In my copending application Ser. No. 704,148, I have described a seismic transducer for marshy terrains which comprises a casing having a bore and a hydrophone slidably mounted in the bore. The casing has in its side wall a plurality of openings. The hydrophone includes an elastomer core in which is completely embedded a pressure detector. The core serves to transmit sound waves impinging through the casing's openings to the detector. A bottom end cap is threadedly coupled to the casing for compressing the elastomer core, whereby portions of the core form plugs which extend through and seal the openings in the side wall of the casing.
It has been found that in order for these plugs to adequately seal the openings in the casing, the end cap must exert a large static pressure on the elastomer core. This pressure establishes a large static load on the detector itself, which has the undesirable effect of appreciably reducing the sensitivity of the detector to seismic waves transmitted through the openings in the casing. If the pressure exerted by the end cap against the elastomer core were to be reduced, so as not to appreciably decrease the sensitivity of the detector, then the plugs formed by the elastomer core would insufficiently seal the openings in the casing.
In U.S. Pat. No. 3,932,834 assigned to the same assignee, there is described a similar transducer for marshy terrains but in which the pressure detector is completely embedded in an elastomer body which completely fills the bore of the casing and which is cemented to the wall of the bore. In use, it was found that the elastomer body would frequently become unglued from the wall of the casing, allowing water to leak through the casing's openings, thereby corroding the electric parts coupled to the detector outside of the elastomer body and in the casing's bore.
The present application describes a seismic transducer embodying the novel structural features generally disclosed in the aforementioned copending application Ser. No. 704,148, the main distinction being that means are now embedded inside the elastomer body for shielding the pressure detector from substantially all the static pressure exerted on the elastomer body, thereby considerably improving the sensitivity of the detector. The applied pressure causes the opposite end portions of the elastomer body to form fluid-tight seals against the wall of the casing's bore. The means inside the elastomer body is a cage which prevents the formation of the aforementioned plugs, so that the seismic waves pass through the openings in the casing and become exerted upon a greater exposed surface of the elastomer body.